The present invention relates to a duct which comprises a number of wall sections produced at least partly from fibre material and a binder agent, which together define a closed, elongate flow space, mutually meeting wall sections being united by the intermediary of at least one longitudinal fold or joint line.
As regards ventilation ducts, increasingly higher demands are placed today on large cross sectional areas, with a view to reducing the flow resistance and thereby energy consumption. As large dimensions as 600-700 mm×300-400 mm have been mentioned as relevant. Further, stringent demands are placed on the ability of the ventilation duct to withstand partial vacuum of at least 200 Pa, as well as partial vacuum surges of 750 Pa. Performance wishes of as large partial vacuum as 1200 Pa have also been raised.
WO 2009/145698 discloses a ventilation duct which is manufactured from a fibre material and a binder agent. The illustrated embodiment is of four-sided cross section, square or rectangular, and as a result has four longitudinal wall sections. Each one of these wall sections displays transverse rigidifying elements in the form of impressions where the quantity per volume unit of fibres and binder agent is considerably higher than that which applies to the rest of the wall sections.
The wall sections are interconnected with one another in the corners of the ventilation duct where fold lines are provided so that the ventilation duct may be flattened out to substantially planar state in order subsequently to be rolled up for compact storage and transport. Cutting of the duct into relevant lengths may also be put into effect most simply in the flattened state of the duct.
The fold lines are in the form of the longitudinal, external impressions in the combined fibre and binder agent material, for which reason the quantity of fibres and binder agent per volume unit is, also here, considerably greater than that which applies to surrounding wall sections.
Because of its relatively slight cross sectional area, the ventilation duct according to the WO publication is self-supporting and can, as a result, be secured in fixing means which are mutually spaced apart quite far, often of the order of magnitude of 2-3 m.
Largely, the above described ventilation duct functions well, but cannot meet with the above-described requirements on large cross sectional areas and the ability to withstand large partial vacuum, and moreover displays certain shortcomings.
Differences or unevenness in the fibre material from which the ventilation duct is manufactured entail that the ventilation duct will have regions which are weaker than neighbouring sections. Such weaker regions will have as a consequence that, on rough handling, the ventilation duet will have a tendency to give way or, in the event of partial vacuum, to buckle inwards at the weakened points. If the ventilation duct has once given way, it will then become even weaker.
When the prior art ventilation duct has been flattened out and rolled up, it has a certain inherent memory when it is once again unrolled and raised. This implies that the ventilation duct has a tendency to become warped, so that its cross section deviates from the intended configuration. The deformed or irregular cross section entails that the ventilation duct becomes excessively sensitive to partial vacuum.
On flattening-out of the prior art duct, large deformations will take place in the fold lines, for which reason the fibres there may be ruptured and be broken away from neighbouring fibres and hinder agent.
On mounting, of the prior art ventilation duct, it occasionally happens that it is subjected to rough treatment, for example is folded crosswise in an undesirable manner. If the ventilation duct has once been folded in this manner, it will be weaker in the folded region, since the fibres and the binder agent between them have been broken.
If the prior art ventilation duct is scaled up to such large dimensions as were disclosed above, which has become increasingly relevant and is employed in connection with large partial vacuums, this implies that the ventilation duct, as was intimated above, will have a tendency to collapse and be compressed. This becomes particularly serious if any of the other drawbacks disclosed above are present at the same time.
As regards ventilation ducts in general, the intention is to keep the flow losses in them as low as possible. This may be achieved by employing ventilation ducts which have large cross sectional areas. The prior art ventilation duct will accept only an extremely limited up-scaling before problems arise, since the corner regions in the duct are of insufficient mechanical strength. The problems become particularly severe if the duct is exposed to partial vacuum or partial vacuum surges. Further, large planar surfaces on the ventilation duct entail greater risk of buckling.
U.S. Pat. No. 3,818,948 also discloses a ventilation duct which includes a fibre material. This ventilation duct also displays a rectangular cross section where the corner regions between mutually meeting, longitudinal walls are joined together and sealed by means of flexible sealing strips.
According to the U.S. Patent Specification, each wait has a number of transverse rigidifying elements, for example consisting of or comprising metal wire. The duct according to the U.S. Patent Specification may also be flattened out to a substantially planar state, and probably also be rolled up for compact storage and transport.
Despite the presence of the transverse reinforcements in the walls of the duct, it lacks the ability to be self-supporting, for which reason it is only suitable for use in short lengths, for example at bends or the like. Nor is it capable of withstanding partial vacuum, since the corner regions are too weak.
It is desirable to design the duct intimated by way of introduction so that the drawbacks considered above and which largely apply to the ventilation duct according to the WO publication are wholly obviated or at least substantially reduced. Otherwise expressed, it is desirable to provide a duct with a large cross sectional area, and moreover a duct which well withstands partial vacuum, even in the form of partial vacuum surges.
It is particularly desirable to design the duct in such a manner that its corner regions and large planar surfaces will have the requisite mechanical strength and rigidity. Further, it is desirable to design the duct in such a manner that it may, in a simple manner, be secured in as retainer or a configurationally defining, element.
According, to an aspect of the present invention, in the fibre material and binder agent of the duct intimated by way of introduction, there are disposed longitudinal rigidifying elements, and that the duct, on its outside, has longitudinal bead-like projections which include the fibre material and the binder agent.
According to one preferred embodiment, the projections are disposed between mutually meeting wall sections.